1. Methods of Controlling NOx Gases in Effluent Gas Streams
In the case of reducing the concentration of the oxides of nitrogen (abbreviated as NOx and consisting generally NO, NO2, N2O, and N2O2), there are numerous existing techniques for controlling the release of this class of pollutants into the air from combustion processes or other processes that generate effluent gas streams. These are principally classified as techniques that are applied at the origin of these gases, such as improving the control of combustion processes and those processes that are applied after the combustion processes to treat the gases after the NOx has been formed.
Techniques that improve the control of the combustion process focus on the elimination of peak flame temperatures over 2000° K that are the primary cause for the formation of NOx. Processes that seek to destroy NOx after it has been formed are generally called post combustion processes and they are used when techniques for controlling NOx formation are not adequate to meet the emission limit goal.
Techniques employed to minimize the formation of NOx in combustion processes generally involve the use of one or more methods of diluting the flame with inert gases that absorb thermal energy and normalize localized combustion temperatures. Methods have been applied that dilute the fuel with steam or water to achieve this dilution and other methods rely on the recirculation of combustion products. In some cases, the existing combustion equipment can be modified to achieve moderate control of NOx emissions using these techniques, and these modifications are typically inexpensive and very cost effective to apply. These include laminar flame combustor that avoids high combustion temperatures by a combination of air dilution and laminar combustion and use of a porous solid combustion catalyst to reduce the flame temperature in gas turbines burning gaseous fuels. This technique has demonstrated the ability to achieve very low NOX levels but long term reliability and engine risk issues have limited the commercial applicability of this product. Combustion modification techniques have demonstrated single digit NOX levels but these techniques have not demonstrated that they can achieve reliable and efficient operation of the combustion device and also achieve NOX emissions below 6 ppm.
Post combustion processes can affect much lower emission levels than are achievable through the use of techniques that minimize the formation of NOx during combustion. Moreover, these processes have been refined over the years to achieve better results and to cost less to implement, and are widely applied. However, pollution control authorities continue to demand even lower NOx emission levels, and post combustion processes are approaching performance limitations caused by physical and chemical constraints inherent to their methods of operation. The result is a disjoint in the NOx, CO and ammonia slip control capability desired by regulatory authorities and the capability which is technically achievable and financially affordable using these processes. This situation produces a need for a different technology that does not have such inherent process constraints because of the manner and method of operation, and therefore can achieve the more aggressive emission limits desired by the regulatory authorities and needed by society to reduce the effects of air pollution.
Post combustion processes include, but are not limited to, selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), or a combination of these processes, called the selective hybrid reduction process (SHR). These processes are applied to gas streams containing oxygen and rely on a reagent to react with NOx in a reduction chemical reaction. The desired end products are water vapor (H2O), diatomic nitrogen (N2), and oxygen (O2). Typical applications include boilers, heaters, furnaces, gas turbines, lean burn gaseous fueled engines, Diesel engines, and dryers. Since oxygen is present in the effluent streams, a reagent is necessary to promote the reduction reaction chemistry. The most suitable reagents discovered so far are ammonia based. Practicable ammonia based reagents include ammonia (anhydrous and aqueous) and urea. Urea decomposes to ammonia in when it is heated above about 700K. Urea is an alternative source of ammonia for the SCR, SNCR, and SHR processes when the on-site storage of ammonia is deemed a safety risk.
a. SNCR Process
In the case of SNCR, the reagent is added exhaust at high post combustion temperatures ranging typically from 1150° K to 1290° K to dissociate ammonia into NH2* and providing the reactant for reducing NOx. No catalyst is involved. But SNCR has application characteristics that limit its performance and practicality for controlling NOx emissions. These characteristics include:                Low reagent utilization efficiency        High carrier gas requirements        High sensitivity to inlet NOx c        
concentration                Dependence on relatively ineffective gas phase mixing of the reagent with NOx molecules        Dependence on narrow temperature range for effective performance        Elevated reaction temperature that is difficult to access in many combustion devices.        
The process typically achieves 50% to 60% NOx reduction performance, but in some cases the performance can be outside this range if the application conditions are particularly good or particularly poor.
The effective temperature window of this process is between about 1150K and 1290K. The initial and final temperatures of this range are not absolute. The temperature window is a function of the gas cooling rate in the apparatus. For example, in an application where the gases cool relatively slowly, such as in a duct with ambient heat losses, the reactivity zone shifts to slightly lower temperatures. In applications where the gases cool rapidly, such as in a boiler, gas temperatures at the upper range of the reactivity zone must be accessed to achieve acceptable NOx reduction performance.
Injection of the reagent at the upper range of the temperature zone causes much of the ammonia to become oxidized by the molecular oxygen in the gas stream to form NOx in a competing reaction. A similar problem at the elevated temperatures is that some of the ammonia will dissociate completely before it has a chance to react with the NOx. If the reagent is injected at even higher temperatures, it is possible to produce more NOx in the gas stream. Similarly, injection at temperatures that are too low compromises the process. At gas temperatures below about 1150° K the ammonia dissociation rate is reduced to the point that too much ammonia will slip by the reaction zone and become released out the stack. For these reasons, this process is sensitive to the effluent gas temperature and cooling rate.
The process is very sensitive to inlet NO concentration which limits its usefulness in some cases. The process is much more effective with high inlet NOx concentrations, but at moderate or low inlet NOx concentrations the process becomes significantly less effective. When the inlet NOx concentrations are in the order of 15-20 ppm the process becomes essentially ineffective, particularly if the combustion apparatus cools the gases at a rapid rate such as would be the case in a boiler.
The method of applying the SNCR process is particularly difficult because of the very high gas temperatures involved. This is particularly a problem in larger combustion devices that have large cross sectional areas. The need to achieve almost instantaneous injection and mixing of the ammonia into the furnace gases of large combustion devices is a difficult challenge and the best methods still consume large amounts of carrier gas and require expensive stainless steel injection hardware.
The SNCR process performance is also very sensitive to NOx—NH3 concentration uniformity and temperature uniformity. At the reaction temperatures involved in this process, gases are typically very turbulent with very non-uniform temperature and gas concentration gradients. Injecting ammonia into gases with these properties results in very poor ammonia utilization. NOx reduction performance is very limited and ammonia slip can be excessive, particularly if the effluent gases are cooling at a rapid rate.
As a result of the difficult application conditions associated with the SNCR Process, this process is generally limited to effluent gas streams from the combustion of very dirty fuels that restrict the use of other post combustion NOx reduction technologies.
b. SHR Process
The SHR process is employed when the SNCR process cannot quite meet the NOx emission goal and there is a technical need to minimize the use of catalyst material used in the SCR process, such as may be caused by space or pressure drop constraints. The ammonia slip from the upstream SNCR process is used in the downstream SCR process to complete the reaction of ammonia with the NOx. This process has very limited application because it has all of the disadvantages associated with the SNCR Process which results in higher operating costs than the SCR Process. It has been applied to combustion devices that do not have physical space for a large SCR catalyst or which cannot tolerate the pressure drop that would be caused by a full sized SCR catalyst.
c. SCR Process
In the case of SCR, the reagent is injected and mixed into the effluent gas stream for reaction with NOx in the solid catalyst bed. Practicable reagents include ammonia and urea. The ammonia is adsorbed into the catalyst matrix and converted to NH2* by the reduction of V+5 to V+4 metal oxide. The optimum operational temperature range is typically from 570° K to 670° K. For temperatures exceeding 690° K, NH2* significantly reacts the molecular oxygen in the exhaust, thus increasing ammonia demand. For temperatures less than 570° K, nitrogen dioxide, NO2, combines with ammonia, NH3, to form ammonium nitrate salt, NH4NO3, which is considered a pollutant in the exhaust gas and the NOx reduction rates in the catalyst become impractically slow. For a near perfect ammonia distribution in the exhaust gas, multiple mixing zones are used in the catalyst bed to achieve 2-3 ppm NOx slip with 5-6 ppm NH3 slip. Multiple mixing zones increase the friction pressure drop of the exhaust gas and size of the catalyst bed, which in turn increases the fan horsepower to push or draw the exhaust gas through the bed.
The physical chemistry of gas-solid interface phenomena results in practical performance limitations of the SCR process such as some of the NOx can pass through the catalyst bed and not collide with NH2* radicals and thereby fail to be controlled by the SCR process, resulting in NOx emissions. A further limitation is that the ammonia may not be evenly mixed in the exhaust gas stream when it reaches the catalyst face, thus causing regions of excess ammonia that result in ammonia slip emissions and/or regions of excess NOx that will not react with NH2* completely and pass out the stack uncontrolled. An added limitation is that the temperature of the effluent gases may not be sufficiently uniform across the catalyst face to cause the catalyst to function optimally or that the bulk gas temperature is either too low or too high for the catalyst to function optimally. Uneven or concentrated gas flow through the catalyst bed can create a region of high effluent velocity. This high effluent velocity can reduce the effectiveness of the catalytic bed by lowering the residence time of the effluent gas to the catalyst. Another limitation of the SCR process is that the catalyst bed can become fouled or poisoned by contaminants in the effluent gas stream and rendered ineffective. Another limitation is that the catalyst bed can fail to be sealed properly in the reactor vessel causing gas by-passing and the release of unreacted ammonia and NOx to the atmosphere.
In addition to the aforementioned functional limitations, the SCR process has other disadvantages to prospective users. These include, but are not limited to, the very large size of the catalyst bed, the cost of the catalyst material, the impact of the pressure drop caused by the catalyst bed, and the magnitude and range of temperature that must be maintained for the process to work properly.
The SCR process requires that the catalyst bed be large enough to achieve a minimum residence time of the reactant gases in the bed in order for the physical chemistry to proceed. This results in large catalyst beds that can often be difficult and expensive to install. The cross sectional area of the bed can be optimized to reduce the bulkiness of the beds, but the required residence times must be maintained and smaller flow areas must be offset by thicker beds. Increasing the thickness of the bed causes a non-linear increase in pressure drop and therefore energy cost to maintain flow. Pressure drops through SCR catalyst beds can range from 1 inch water column to more than 5 inches water column. This pressure drop consumes energy from the gas turbine to enable effluent stream flow rate to be maintained.
Catalyst beds associated with the SCR process are also very heavy and this results in the need for significant structural capability of the apparatus and support foundations. The weight also requires heavy lifting equipment during construction. These requirements further add to the installation cost. The materials used to coat the catalyst substrate are rare alloys that are expensive to buy and apply and therefore cause the catalyst material to be undesirably expensive.
The SCR process is also very temperature dependent and it is often necessary to make such modifications as necessary in the equipment employed to use the effluent stream so as to make access to the correct temperature window for the SCR reaction. The correct catalyst temperature window can range from less than 477° K to as much as 700° K depending on the catalyst material used. This temperature range is normally found in the middle of a thermal device employed to recover energy from the effluent stream, and access requires the alternation of the heat recovery device at the precise location to enable access to the correct SCR operating temperature. Furthermore, if the heat recovery device is a boiler used for the production of steam, the correct temperature may be within the evaporator tube bundle, whereupon the creation of an installation cavity would be expensive and may cause boiler circulation problems. Therefore, the SCR process is known to have significant physical and chemical shortcomings that cause the SCR process to be understandably expensive to install and use, and also to have limited NOx reduction performance capability.
In these processes, ammonia dissociates into NH2*+H on the surface of the catalyst which is a reducing agent to NOx. NH2* and absorbed NOx molecules collide and react to form N2 and H2O. NO2 helps reset the catalyst sites by reacting with the absorbed H to form water. The overall chemical reactions are given by:NO+NH2*=N2+H2O and NO2+2H=NO+H2O
All three processes are presently in active use, and the selection of the most appropriate process is generally made on the basis of the lowest evaluated cost method of meeting the NOx emission goal. The least expensive and also the least effective process is the SNCR process and the most effective and more expensive
NOx reduction technology is currently the SCR process equipped with an ammonia destruction catalysts.
2. Methods of Controlling Combustion Contaminants and VOC's
VOC's are a general class of compounds that are organic and that have vapor pressures that are low enough to allow them to become airborne. More specifically, as applied to the science of air pollution, these compounds have properties that cause them to be toxic to plants and animals in their raw state or that are precursors to air pollutants that become formed in the atmosphere. These compounds are of significant interest to the public and to air quality regulators responsible for air quality maintenance and their release into the atmosphere is therefore regulated.
There are thousands of VOC's made by man and also thousands of processes that can cause the accidental release of these compounds into the atmosphere. Some examples include painting and coating operations, printing, chemical processing, food processing, baking, fast food broilers, and any process that generates odors.
The variability in the amount and concentration of these compounds released by various anthropogenic sources resulted in the development of different types of technologies for their abatement. But all of these technologies can be classified into fundamental process methods, absorption, thermal or catalytic oxidation, adsorption, refrigeration, biological oxidation, and photochemical oxidation.
a. Absorption
Absorption methods for controlling VOC's consist of scrubbing the effluent gas stream with a chemical solution that absorbs the organic compounds. The scrubbing solution can be water or a water based solution or it maybe another organic compound such as a glycol based solution. In water scrubbers, the VOC rich effluent liquid stream maybe disposed of directly or removed from the scrubbing fluid by adsorption or precipitation. In the case of scrubbing processes that use an organic compound to absorb the VOC's, the VOC rich effluent liquid stream is typically heated to desorb the VOC's and then recirculated back to the reactor. In some cases the VOC's can be converted to useful by-products but often the VOC's end up in wastewater for direct disposal into city sanitary sewers or water treatment facilities.
Scrubbers are effective for very few VOC compounds and for that reason, are not widely used for VOC emission control. Most VOC's are not absorbent in water. Hydrocarbons can be readily absorber into glycol or mixtures of glycol and alcohols and effluent streams laden with these compounds are often treated by this method.
Since gas scrubbing relies on a gas-to-liquid phase interaction, the efficiency of the process is constrained by mixing and boundary layer effects, and high removal efficiencies are difficult to achieve. The removal efficiencies can be improved by increasing the energy intensity of the scrubbing process but the higher energy cost and decreasing absorption gains limit the cost effectiveness of this approach.
The process of scrubbing VOC's from effluent streams transfers the VOC's from the effluent gas stream to the scrubbing liquor, and the purified gas stream is released into the atmosphere or to use in some other process. The gaseous organic compounds then must be removed from the scrubbing liquor so that the liquor can be recycled to the reactor. In closed cycle applications the rich scrubbing liquor is fed to a desorber column where it is heated and or reduced in pressure by vacuum means to draw out the effluent. The effluent is the condensed or incinerated depending on the nature and value of the particular compound involved.
Thermal oxidizers handling VOC materials that contain chlorine, fluorine, or bromine atoms generate HCL, CL2, HF, and HBr as additional reaction products during oxidation, and scrubbers are applied downstream of the thermal oxidizers to remove these contaminants.
b. Thermal Oxidation
The process of thermal oxidation raises the effluent gas stream several hundred degrees above the autoignition temperature of the VOC's in the stream that are to be destroyed. The products are retained at high temperatures for a specific length of time, called the residence time, to achieve high VOC destruction rates. The residence times may range from a fraction of a second to over two seconds.
Thermal oxidizers operate at high temperatures, typically between 1000° F. to 1600° F. and a fuel is burned to furnish enough thermal energy for the VOC's to oxidize. Recuperative heat exchangers are employed to reduce fuel consumption and these heat exchangers can be designed to be nearly 70% effective in reclaiming heat. Destruction efficiencies depend on many factors including the stability of the organic compounds that are being destroyed and the design parameters of the thermal oxidizer, but a properly designed and operated thermal oxidizer can achieve 99% destruction efficiency.
Thermal oxidation is employed when the effluent stream contains sulfur, halogenated compounds, or some metals such as lead, phosphorus, zinc, or tin, or if they are extremely stable and difficult to destroy. However, the incineration of VOC's that contain chlorinated compounds can create the formation of dioxins and furans if the combustion process is not complete.
A significant disadvantage of thermal oxidation is that it creates NOX from the high temperature combustion process. Also, to avoid the danger of explosion when using incineration techniques, the concentration of the VOC materials should total less than 25% of the lower explosion limit (LEL).
c. Catalytic Oxidation
The current art for controlling some VOC's is a process called catalytic oxidization. Catalytic oxidation is a process involving the following steps:                The effluent gas stream is drawn into a heat exchanger to preheat the effluent stream by hot exhaust gases from the oxidization process.        The preheated effluent stream is drawn into a combustor and mixed with the products of combustion from an auxiliary burner burning an external fuel such as natural gas.        The effluent stream is mixed with the combustion products and drawn into a catalyst bed and temperature controlled to the optimal temperature for the catalyst bed.        The catalyst material, typically a noble metal catalyst material such as platinum or palladium, promotes the oxidation of the VOC's.        The purified effluent stream is drawn into the heat exchanger used to preheat the inlet effluent stream and blown to the atmosphere with an induced draft fan.        
Catalytic oxidizers can operate in a temperature range between about 530° K to 800° K, but more typically between 550° K to 620° K. Destruction efficiencies can be as high as 99.9%. They are in common use for destroying many types of VOC materials because they use less fuel than thermal oxidizers but they do have certain disadvantages: they cannot be used on sources that also generate small amounts of catalyst poisons such as tin, phosphorus, zinc, and lead. Also, they are vulnerable to chemicals or particulate matter that can mask or foul the catalyst surface.
For effluent stream containing dilute concentrations of VOC contaminants, incineration techniques are an expensive control method just due to the large fuel cost. Current best art for effective control of such effluent streams involves the adoption of a pretreatment step to concentrate the VOC's so that the amount of gas that must be heated is smaller.
VOC concentrators consist of absorbers that strip the VOC's from the effluent gas stream and discharge the purified effluent stream. The adsorbers can strip off the VOC's until they saturate and then they must be switched over for VOC desorption. In the VOC desorption stage, clean air mixed in with incineration off gas is used to drive the VOC's.
d. Adsorption
Adsorption is the process that involves the adherence of gaseous molecules into porous materials. A familiar adsorption process is the use of activated carbon to remove chemicals and odors from water. Activated carbon is also the primary adsorbent material used for VOC removal in effluent gas streams. An absorption system used for removal of VOC materials would contain beds of carbon situated in a reactor in a manner such that all of the effluent gas stream must pass through the adsorbent material.
Adsorbent technology is used when the effluent stream only contains one to three specific organic solvents and that it is economical to recover these solvents or when the VOC concentration is very dilute and it is desired to pre-concentrate the VOC's for thermal or catalytic oxidation.
Adsorption technology is typically most effective on sources that generate organic compounds having a molecular weight of more than 50 and less than approximately 200. The low molecular weight compounds adsorb poorly and the high molecular weight compounds adsorb so well that they can be very difficult to desorb to allow the adsorbent to be reused.
Adsorbent systems are not recommended for streams that contain particulate matter and/or high moisture concentrations because they compete for the pore spaces on the adsorbent and this reduces the number of pores available for the VOC material.
e. Refrigeration
Condensation, refrigeration, and cryogenic systems remove organic vapor by making them condense on cold surfaces. These cold conditions can be created by passing cold water through an indirect heat exchanger, by spraying cold liquid into an open chamber with the gas stream, by using a Freon-based refrigerant to create very cold coils, or by injecting cryogenic gases such as liquid nitrogen into the gas stream. The concentration of VOC's is reduced to the level equivalent to the vapor pressures of the compounds at the operating temperature. Condensation and refrigeration systems are usually used on high concentration, low gas flow rate sources. Typical applications include gasoline loading terminals and chemical reaction vessels.
The removal efficiencies attainable with this approach depend strongly on the outlet gas temperature. For cold-water-based condensation systems, the outlet gas temperature is usually in the 40 to 50° F. range, and the VOC removal efficiencies are in the 90 to 99% range depending on the vapor pressures of the specific compounds. For refrigerant and cryogenic systems, the removal efficiencies can be considerably above 99% due to the extremely low vapor pressures of essentially all VOC compounds at the very low operating temperatures of −70° F. to less than −200° F.
Condensation, refrigeration, and cryogenic systems are usually used on gas streams that contain only VOC compounds. High particulate concentrations are rare in the types of applications that can usually apply this type of VOC control system. However, if particulate matter is present, it can accumulate on heat exchange surfaces and reduce heat transfer efficiency.
f. Biological Oxidation
Biological systems are a relatively new control device in the air pollution control field. VOC's can be removed by forcing them to absorb into an aqueous liquid or moist media inoculated with microorganisms that consume the dissolved and/or adsorbed organic compounds. The control systems usually consist of an irrigated packed bed that hosts the microorganisms (biofilters). A presaturator is often placed ahead of the biological system to increase the gas stream relative humidity to more than 95%. The gas stream temperatures are maintained at less than approximately 105° F. to avoid harming the organisms and to prevent excessive moisture loss from the media.
Biological oxidation systems are used primarily for very low concentration VOC-laden streams. The VOC inlet concentrations are often less than 500 ppm and sometimes less than 100 pm. The overall VOC destruction efficiencies are often above 95%.
Biological oxidation systems are used for a wide variety of organic compounds; however, there are certain materials that are toxic to the organisms. In these cases, an alternative type of VOC control system is needed.
g. UV Oxidation
One method of using UV energy to activate the oxidation of VOC's involves the use of hydrogen peroxide and is currently being tested. The requirement to use hydrogen peroxide has been determined to be a significant safety issue and cost and so the current art of the use of ultraviolet energy is not commercially viable.
Another technique relies on UV emitters that have wavelengths greater than 250 nm and that cause the dissociation of nitrogen molecules, and thereby promoting the formation of NOx.
The SUVR process for VOC destruction is a more cost effective approach to controlling VOC pollutants than thermal oxidation because it does not require the use a fuel or fans to overcome the pressure drop caused by the incinerator and heat exchangers. It is does not require the use of a precious metal catalyst such as in the catalytic oxidation process nor is it limited in the gases it can treat such as catalytic oxidation.
The SUVR process will not displace adsorption and refrigeration processes that are applied to effluent gases to remove and recycle the VOC's because the SUVR process is an oxidation process and destroys the organic solvents.
3. Description of Prior Art
Further in regard to the above, thermal NOx (NO, N2O, NO2), CO, VOC's partially oxidized hydrocarbons (CxHxOx), SOX (SO2, SO3) from mobile and stationary combustion sources are major air pollutants in high-density urban areas. Air pollution remains a problem in these urban areas and environmental regulatory organizations continue to develop new regulations to force more effective and lower cost means for reducing such emissions.
In the case of NOx emissions, there are three major classifications of post combustion NOx removal methods. These are: reducing NOx to N2 with a reactant, oxidizing NOx into nitrogen acids (HNO2, HNO3) with wet/dry scrubbing, and direct absorption on a solid. The method of reducing NOx to N2 by means of using a reactant has become the most cost effective and highly developed means applied in today's marketplace and for all practical purposes represents current state-of-the-art. There are three sub-classifications of the NOx reducing methods in general use today; Non-Selective Catalytic Reduction (NSCR), Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR).
Oxidizing NOx into nitrogen acids with wet/dry scrubbing appears economical only for large stationary combustion sources where there is a fertilizer demand, or where the fuel has high sulfur or ash content. This wet/dry scrubbing process is described in U.S. Pat. Nos. 6,605,263 and 6,676,912. U.S. Pat. No. 6,605,263 describes how to inject ammonia to convert SOx into an ammonium salt, (NH4)2SO4, which is then removed with a wet scrubber. U.S. Pat. Nos. 6,676,912 and 6,651,638 describe injecting hydrogen peroxide into the gas stream to oxidize NOx to nitrogen acids and remove the acids with wet scrubbing.
U.S. Pat. No. 6,523,277 discloses injection of hydrogen peroxide into the exhaust gas and activates it with microwave radiation to oxidize NOx to nitrogen acids that can be removed with a wet scrubber. U.S. Pat. No. 6,743,404 demonstrates how to decompose N2O contaminant gas into nitrogen (N2) and oxygen (O2) gases using a group II metal oxide catalyst (CuO and ZnO on Al2O3).
U.S. Pat. No. 6,612,249 discloses decomposition of NOx and sequesters mercury and other heavy metals with injection of magnetite in the exhaust gas stream and recovering the product in the ash. U.S. Pat. No. 6,488,740 describes how to use a wet electrostatic precipitator to remove acid gases (HNO3) and coal ash. U.S. Pat. No. 5,843,210 discloses use of an electrostatic spray with scrubber.
Another method of reducing NOx and SOx in exhaust gases is to adsorb them on a high surface area bed. U.S. Pat. No. 6,506,351 describes how to absorb NOx and then oxidize the NOx compounds with ozone to N2O5, which can be removed with wet scrubbing during the regeneration cycle. U.S. Pat. No. 6,503,469 describes how to absorb volatile organic compounds and NOx in exhaust gas on high-silica adsorbent and oxide them with ozone. U.S. Pat. No. 6,066,590 employs a manganese oxide and ruthenium chloride based adsorbent to oxidize NOx and SOx to acids, and reacts the acids with an alkali metal (Ca or Mg) to produce a solid salt that remains in the filter.
NSCR methods for mobile combustion sources typically use unburned or oxygenated hydrocarbons, CO, or a reducing agent in the fuel itself to reduce NOx emissions to N2. As shown in U.S. Pat. Nos. 6,742,326, 5,524,432 and 5,336,476, the fuel/air ratio must be controlled to slip enough reducing agent into the catalyst to allow the NOx to be reduced to N2. As shown in U.S. Pat. No. 6,725,643, for a diesel engine, water with an amine compound can be emulsified with the fuel to reduce NOx to N2 in the cylinder. As shown in U.S. Pat. No. 6,682,709 ammonia or cyanuric acid can be injected down stream of the engine and activated by burning additional fuel to decompose the NOx. As shown in U.S. Pat. No. 6,612,249, for a stationary combustion source, iron flakes can be used in the combustion chamber to reduce NOx to N2 and absorb mercury vapor. As shown in U.S. Pat. No. 6,224,839, NOx in exhaust gas can be reduced to nitrogen by reaction with an activated carbon bed impregnated with an alkali metal. The technical challenge is preventing excess oxygen from reacting with the activated carbon to produce carbon monoxide.
SCR methods for stationary lean combustion sources normally employ ammonia, NH3, or urea to reduce NOX emissions to N2. The solid catalyst surface reduces the activation energy required to reduce NOx with NH3 to produce N2 and H2O by decomposing NH3 to the NH2* radical or by absorbing NOx on the surface to react with the ammonia. U.S. Pat. Nos. 5,744,112 and 5,670,444 describe SCR catalyst compositions of a mixture of precious metals deposited on a ceramic support structure. The solid catalyst is selective for NOx reduction and not for CO oxidation; therefore CO will remain in the exhaust emission. Since the catalytic reaction requires a slight excess of ammonia ion, NH3, compared to NOx molecules, there is usually a little ammonia slip in the exhaust. The current state of the art for use of NOx and NH3 slip for high temperature combustion, using a low exhaust gas recirculation and a SCR with or without SNCR system, is about 2 to 3 ppm for NOx and 5 to 7 ppm for NH3 gases, and for older systems the slip can range from 10 to 25 ppm for both gases. U.S. Pat. No. 6,287,111 uses staged exhaust gas recirculation to reduce NOx generation by employment of a dual-staged burner system. The current state of the art for NOx generation by means of burners using high exhaust recirculation and low temperature combustion is about 5-9 ppm NOx. Thus, there is still a need for final polishing of the exhaust to reduce ammonia slip and NOx below 1 ppm NOx to create near zero emission combustion sources.
U.S. Pat. No. 6,739,125 discloses use of SCR on a mobile engine while supplying ammonia made from the rich fuel and air mixture.
U.S. Pat. No. 6,730,125 discloses production of ammonia from the hydrolysis of urea and use of the ammonia in an SCR based NOX treatment process. By injecting ammonia instead of urea, the residence time is reduced in the SCR unit and the NOx conversion is more complete.
U.S. Pat. No. 6,550,250 demonstrates how to make use of an aqueous solution of urea injected into the hot exhaust gases to make ammonia for an SCR. U.S. Pat. No. 6,146,605 uses a combined SNCR/SCR process where urea is thermally decomposed into ammonia upstream of the SCR.
Another NSCR method uses an ultraviolet photocatalyst to remove ammonia and nitrogen monoxide from atmospheric gases. U.S. Pat. No. 6,566,300 discloses use of titanium oxide on a zeolite carrier as a UV photo-catalyst to decompose ammonia and adsorb NO gas. U.S. Pat. No. 6,562,309 discloses use of a titanium-oxide based, UV photo-catalyst bed to oxidize hydrocarbon fuel vapors.
U.S. Pat. Nos. 6,468,489 and 6,267,940 discloses injection of a UV photo-catalyst powder (powdered SCR catalyst) and ammonia in the exhaust gas, then exposing the exhaust gas mixture to UV light to reduce NOx. The powder catalyst is recovered and recycled. U.S. Pat. No. 6,153,159 discloses use of a UV photo-catalyst fluidized bed to reduce NOx to nitrogen gas while oxidizing unburned hydrocarbons.
U.S. Pat. No. 6,346,419 uses filtered UV light from a mercury lamp (365 nm band pass filter) to disassociate NO2 to NO for the chemiluminescence's detection.
Another NSCR method uses non-thermal plasma in a catalyst bed to oxidize diesel soot or unburned hydrocarbons to carbon dioxide and reduce NOx to nitrogen gas, as shown in U.S. Pat. Nos. 6,475,350, 6,038,854 and 5,711,147. U.S. Pat. Nos. 6,395,238 and 6,139,694 demonstrate how to use a non-thermal plasma and ethanol injection to oxidize NOx to nitrogen dioxide (NO2) gas, which is removed with a wet scrubber as nitric acid (HNO3). U.S. Pat. No. 6,365,112 uses a corona discharge to decompose water vapor and oxidize unburned hydrocarbons, NOx and SOx so a wet electrostatic precipitator can remove them. Ammonia or urea can be mixed with the water vapor to help reduce NOx to nitrogen (N2) gas.
U.S. Pat. No. 6,345,497 uses an electron beam and U.S. Pat. No. 6,030,506 uses a hollow cathode discharge to create atomic nitrogen to inject into the exhaust gas stream to reduce NOx to nitrogen (N2) and oxygen (O2) gases. The technical challenge for large installations is to mix the atomic nitrogen with the exhaust gas in less than 10 to 14 milliseconds.
U.S. Pat. No. 6,176,078 uses a non-thermal plasma to partially oxidize fuel to create a hydrocarbon based reducing agent for an SCR NOx reduction system. An oxidization catalyst is used down stream of the SCR to oxidize any hydrocarbons not used to reduce NOx to nitrogen gas. U.S. Pat. No. 6,117,403 uses a barrier discharge or electron beam to oxidize NOx and SOx to acids and then employs a wet scrubber to remove them from the exhaust. U.S. Pat. No. 6,773,555 uses an electron beam to oxidize NOx and SOx to nitric and sulfuric acids, neutralize the acids with ammonia gas to make ammonium salts, and then remove the salts with a dry scrubber. U.S. Pat. No. 6,027,616 uses corona discharge and Helium and an oxygen gas mixture to produce O2+ ions, which oxidize NOx to nitrogen acids to be removed with a wet scrubber.
U.S. Pat. No. 6,264,899 injected hydroxyl radicals and atomic oxygen upstream and downstream of the combustion process to improve the VOC reduction in a catalytic converter. U.S. Pat. No. 5,154,807 doped the fuel with hydroxyl radicals and zinc and injected hydroxyl radicals downstream of the combustion process to reduce the VOC contamination in the exhaust gas. U.S. Pat. No. 5,807,491 used an electron beam generation of hydroxyl radicals from liquid water to clean VOC's from contaminated water or air. U.S. Pat. No. 6,811,757 uses a dielectric barrier discharge to create hydroxyl radicals and atomic oxygen to remove VOC's in the contaminated air. U.S. Pat. No. 5,236,672 uses a pack bed corona generator to generate hydroxyl radicals and atomic oxygen to inject into a contaminated air stream to reduce VOC's.
U.S. Pat. Nos. 6,682,709 and 4,448,176 use preheated fuel to burn under super lean conditions (equivalence ratio less than 0.6) to reduce VOC and NOx emissions. U.S. Pat. No. 6,345,495 uses preheated saturated moist air-fuel mixture under super lean conditions and a combustion catalyst to create flameless hydroxyl radical combustion to reduce VOC and NOx generation by keeping the combustion temperature below 1200 K. U.S. Pat. No. 5,876,195 used a laser to preheat and ignite the fuel charge. U.S. Pat. No. 4,885,065 used an electron or ion bean to sustain combustion under super lean conditions (equivalence ratio less than 0.6) by creating hydroxyl radicals from the fuel and oxygen in the combustion zone. U.S. Pat. No. 5,487,266 used flame spectroscopy to adjust fuel mixtures to lower flame temperatures to reduce the NOx emissions without causing flame out in the gas turbine.
None of the above techniques provide the unusually effective method of NOx reduction and simultane oxidization of combustion contaminants (CO and/or CxHxOx) which are now afforded by the present invention.